The invention relates to aeronautics, and more specifically relates to systems that remove ice from aircraft surfaces. In its most immediate sense, the invention relates to ice-protection systems for use on airfoils that are roughness-sensitive. As used herein, "roughness" refers to "sandpaper" ice, distributed ice contamination or spanwise steps or ridges of ice downstream of the leading edge.
To be commercially successful, a new airplane must be fast, fuel-efficient, and capable of operation in icing conditions. For these reasons, airframe makers have devoted intensive attention to developing advanced designs for lifting and control surfaces. This effort has produced high performance aircraft that may perform or handle poorly under certain icing conditions, and particularly under icing conditions that produce "sandpaper ice".
When a lifting or control surface encounters supercooled water in an "inadvertent encounter" of one or two minutes duration, the ice accretes in a characteristic pattern. The accreted ice has a low height and a rough texture; the texture is sufficiently rough to adversely affect the airflow over the ice. Such ice is known as "sandpaper ice". Ice roughness can be left on surfaces when a de-icing system fails to remove all the accumulated ice, thereby leaving a residue of ice that may be unacceptable.
Sandpaper ice accretion on lifting surfaces (e.g. wings and tails) and particularly at and near the leading edge of a wing, roughens the surface of the wing and changes the airflow pattern over and around it. If the lifting surface is sensitive to roughness, this change in airflow pattern substantially degrades the maximum lift and the angle at which maximum lift occurs.
So, too, roughness upstream of control surfaces (e.g. ailerons and elevators) can interfere with the desired operational characteristics of those surfaces. This is because such ice accretion disturbs the airflow and creates a separation bubble. When the thus-disturbed turbulent airflow reaches the control surfaces downstream, it can dramatically alter the hinge moments of such surfaces, even to the extent of reversing such moments.
Accordingly, to perform properly in icing conditions, the leading edges of roughness-sensitive surfaces must be kept almost entirely free of ice at all times, and ice ridge steps downstream of the leading edge must be minimized.
Evaporative anti-icing systems have been used to keep surfaces free of ice. An evaporative anti-icing system works by transferring sufficient heat to impinging water to cause the water to evaporate. Because of this evaporation, there is no ice or liquid water to flow beyond the boundary of the heated region of the airfoil and to thereby accrete as ice on the unheated region.
In an evaporative anti-icing system of the "bleed air" or the "hot gas" type, bleed air drawn from the engine compressor, or hot gas drawn from some other source, is used as a heat source. Such systems are unsatisfactory when bleed air or hot gas is unavailable in sufficient quantities and/or when it is impractical to deliver bleed air or hot gas to the surface to be protected. In such instances, evaporative electro-thermal systems are logical alternatives.
For an evaporative electro-thermal system to keep the leading edge of an airfoil entirely ice-free, the system requires substantial electrical power. Such a power requirement could be satisfied, if at all, only at great expense. For this reason, an evaporative electro-thermal anti-icing system would be impractical on aircraft with roughness-sensitive surfaces.
If ice is allowed to build up, the accreted ice can be removed using a mechanical de-icing system. In such systems, accreted ice is periodically removed by mechanically deforming (as by a rubber boot or an electrically operated mechanical actuator) the surface upon which the ice accretes. Such systems can only work once the ice layer has accreted beyond some critical thickness. If the layer of ice has not reached its critical thickness, it will merely deform together with the surface upon which it has accreted and will not be removed. This is particularly true when a "brief encounter" causes sandpaper ice to form on an initially clean surface. Sandpaper ice is difficult to remove using any means other than heat. For this reason, a mechanical de-icer system would likewise be impractical on aircraft with roughness-sensitive surfaces.
A hybrid anti-icing and de-icing system is also known. In this system, an electro-thermal "running wet" anti-icer is located at the leading edge and covers the stagnation region for the full envelope of the aircraft. An electro-thermal de-icer is located aft of the anti-icer. The running wet anti-icer (usually called a "parting strip" to indicate its function of preventing an ice cap from accreting on the leading edge) heats impinging water only to such an extent that freezing is prevented. Thus, the water neither fully evaporates from, nor freezes on, the parting strip. The heated and as yet unfrozen water runs back aft of the parting strip onto the de-iced surface, where the heated water eventually loses its heat and freezes. The frozen water is then periodically removed by the electro-thermal de-icer. (The electro-thermal de-icer may have a plurality of de-icer segments downstream of the parting strip, each one removing ice that has formed aft of the one before it.)
This system, too, is unsuitable for aircraft with roughness-sensitive surfaces. When the electro-thermal de-icer sheds the ice accreted aft of the leading edge, there is necessarily some water runback to surfaces aft of the de-icer, which surfaces are not de-iced. Such water runback freezes aft of the de-icer, and forms ridges of ice. If an airplane is e.g. in a holding pattern during icing conditions, ice ridges can continue to grow taller until they substantially degrade the lifting and control performance of the wings and control surfaces.
Therefore, known ice-protection systems are not always adequate for use on aircraft surfaces that are roughness-sensitive. Likewise, known ice-protection systems are not adequate for use immediately upstream of control surfaces that function improperly in the presence of separated airflow.
One object of the invention is to provide an ice-protection system that would adequately anti-ice surfaces that are roughness-sensitive, or that are sensitive to residual ice (e.g. ice ridges) left by operation of existing ice protection systems.
Another object is to provide such an ice-protection system that would perform its function without unreasonably large requirements for electrical power or bleed air.
Still a further object is to provide such an ice-protection system that would be economical and easily adaptable to different applications.
Yet another object is, in general, to improve on ice-protection systems of this general type.
These objects, and others that will be apparent hereinafter, are achieved in accordance with the invention. The invention proceeds from the realization that there is a particular combination of individually known ice-protection systems that performs unexpectedly well. This combination is a hybrid system using a running-wet anti-icer at the leading edge region, and using a mechanical de-icing apparatus aft of the anti-icer. Advantageously, the anti-icer is electro-thermal or of the hot gas type and is located within the roughness-sensitive zone of the airfoil or surface, and the mechanical de-icing apparatus is advantageously of a new type wherein an aerodynamic surface of the airfoil is formed by a semi-rigid skin and the skin is flexed by at least one mechanical actuator.
Using a running-wet anti-icer at the leading edge region is more thermally efficient than using an evaporative anti-icer, thereby reducing the power needed to run the system. Furthermore, using mechanical de-icing aft of the anti-icer does not cause ice ridges to build up aft of the de-iced surface, because there is no water runback. As a result, use of mechanical de-icing apparatus aft of a running-wet anti-icer achieves a substantially anti-iced result without undue expenditure of money and energy.
Especially advantageous results are obtained when the mechanical de-icers are of the new type in which part of the aerodynamic surface is a semi-rigid skin that is flexed by at least one mechanical actuator. This type of de-icer can shed accreted ice layers as thin as 0.050 inch. By cycling the mechanical de-icers at short intervals during the icing encounter, the ice ridges downstream of the leading edge can be reduced to an acceptable height and the surfaces aft of the leading edge can thereby be maintained in a satisfactorily ice-free condition. As a result, the surface is, in effect, anti-iced without the power consumption ordinarily associated with anti-icing.